Dan G. Allen

Single shot high resolution THz upconversion spectrometer

We have designed an upconversion spectrometer that has been used to measure the frequency of individual pulses from the UCSB free electron lasers (FEL). The FEL THz radiation is mixed with NIR radiation in a non-linear optical crystal to generate sum and difference frequency radiation. The FEL frequency is obtained by measuring the frequency difference between the fundamental radiation and the sum or difference frequency sideband, which is accomplished using a 1 m focal length Czerny-Turner spectrometer. Single shot sum and difference frequency generation of NIR and THz radiation is demonstrated with accuracy up to 0.1 cm-1 and with signal to noise ratios of greater than 100:1.

A diffraction-compensating 0–25ns free space terahertz delay line for coherent quantum control

Free space delay lines provide pulses of variable time spacing for optical experiments such as pump-probe spectroscopy and coherent quantum control, including spin and photon echo techniques. However, in the terahertz region of the spectrum, beam divergence due to diffraction limits the useful length of traditional free space delay lines. We present a novel double-folded variable delay line for light in the frequency range 0.24-1.2 THz, which incorporates a symmetric arrangement of lenses whose spacing can be adjusted to compensate for diffraction at each delay. Scalable for use in other wavelength regimes, the design relays an input Gaussian beam waist to the output with up to 25 ns ( approximately 8 m) total delay and is enclosed in a desiccated volume of <0.5 m3. The delay line can deliver two or three pulses with relative amplitudes controlled via variable spacing silicon etalon beam splitters. Profiles of a 0.24 THz beam show good agreement with calculations at long delays, with insertion loss per delay stage of approximately 3 dB.

Optical detection of THz-induced strong field effects in ensembles of neutral donors

Narrowband THz radiation drives transitions between bound electron states in GaAs neutral donors. Elastic light scattering from a donor bound exciton resonance allows time-resolved measurements of the excited state lifetime and THz-induced AC Stark effect.

High fidelity optical readout of excited-state lifetimes and ionization of hydrogenic donors in GaAs

Optical detection of terahertz-frequency Rydberg transitions of neutral donors in GaAs is demonstrated via state-dependent resonance fluorescence from the neutral donor bound exciton (DX0) transition. The intensity of the elastically scattered light is proportional to the fraction of donor-bound electrons in the (bright) ground state; excited state and ionized donors are dark. Resonance fluorescence recovery after terahertz excitation yields a single-shot, time-resolved measurement of the lifetime of the lowest excited (2P−) state. We also demonstrate contactless detection of electron ionization/capture via the ionized donor exciton (DX+) transition.

Lifetime measurements of excited neutral donor states in GaAs detected by resonant elastic light scattering

Recovery of electrons from excited neutral donor states in GaAs is time-resolved using a novel optical readout technique and pulsed terahertz excitation. Long (0.1-1 mus) 2P state lifetimes are measured at various magnetic fields.